The present invention relates to a fuel assembly for a light water reactor, and more particularly to a fuel assembly to be charged in a boiling water reactor.
Recent increasing interests are in longer operating cycles of light water reactors and higher burnup levels of uranium fuel, because of an increase in economical merits such as reduction in discharge of spent fuel, power generation cost, etc. Particularly in Japan, the atomic power generation is based on reprocessing of spent fuel as its premise, with keen requirements for higher burnup level, including reutilization of plutonium extracted by the reprocessing. The nuclear fuel is now discharged from the reaction at a burnup level (discharge burnup level) of about 30 GWd/t, and when a nuclear fuel can have a discharge burnup level of 60 GWd/t, the economical merit will be much improved. In order to attain a higher burnup level in light water reactors, it has been so far tried to improve the corrosion resistance of materials of members for a fuel assembly, prevent deformation of members of a fuel assembly in a neutron irradiation circumstance, optimize the enrichment and arrangement of uranium fuel, and improve the thermohydraulic characteristics of a fuel assembly.
A higher corrosion resistance is required for materials for a fuel assembly of high burnup level than for the conventional materials. As materials of members for the fuel assembly, a zircaloy (Zry: Zn--Sn--Fe--Cr--Ni alloy having the following composition: Sn: 1.2-1.7 wt. %, Fe: 0.07-0.24 wt. %, Cr: 0.05-0.15 wt. %, Ni: &lt;0.08 wt. %, the balance being Zr and impurities) is now used. On the zircaloy members of a fuel assembly, local corrosion called "nodular corrosion" develops in the prevailing circumstance of the boiling water type, light water reactor (BWR). To prevent such a corrosion, processes of improving the corrosion resistance of zircaloy by heat treatment, for example, by heating it to an (.alpha.+.beta.) phase or .beta.-phase temperature region for a short time, followed by quenching, have been proposed (Japanese Patent Publications Nos. 61-45699 and 63-58223). Furthermore, a technique of improving the corrosion resistance by changing the alloy composition is known. For example, a zircaloy having higher Fe and Ni content is known [Japanese Patent Applications Kokai (Laid-open) Nos. 60-43450 and 62-228442].
The Zr alloy material is used in locations subjected t neutron irradiation and thus undergoes irradiation growth and deformation. Particularly, when a curving deformation or expansion deformation takes place at the channel box (FCB), clearances between FCB and control rod are decreased (e.g. to zero), resulting in nuclear reactor operation troubles. To prevent the deformations, a process for suppressing the irradiation growth by making the crystallographic orientation parameter, in the FCB longitudinal direction of (0002) face of hexagonal Zr crystal of 0.15 to 0.5, has been proposed in Japanese Patent Application Kokai (Laid-open) No. 59-229475.
In a boiling water type nuclear reactor, cooling water flows into clearances among fuel rods from the lower tie plate of a fuel assembly and is heated and boils, while passing through the clearances among the fuel rods from the bottom position upwards, to form a two-phase stream of steam voids and liquid water which flows out through through-holes of the upper tie plate. The void ratio is 0% at the bottom position of the fuel assembly and reaches about 70% at the top position. That is, a ratio of hydrogen atoms (H) to heavy metal atoms (U) (H/U ratio) differs between the bottom and the top of the fuel rods. At the bottom position of a fuel assembly, where the H/U ratio is high, the average neutron energy is lowered and the fission reaction of thermal neutrons with nuclear fuel substance is promoted, whereas at the top position of the fuel assembly where the H/U ratio is low, the fission reaction of neutrons with the nuclear fuel substance is suppressed. As a result, the linear heat rating is higher at the bottom position of the fuel assembly than at the top position of the fuel assembly, resulting in uneven power distribution in the axial direction of the fuel rods. Uneven power distribution occurs even in the radial direction of the fuel assembly. The outermost periphery of a square lattice arrangement of 8.times.8, 9.times.9 or 10.times.10 fuel rods is surrounded by an FCB to form a water gap between the outermost periphery of the fuel rods and the adjacent FCB. That is, the H/U ratio is higher at the outermost peripheral region of a fuel assembly than at the inner region thereof, and thus the linear heat rating will be higher. To attain a longer operating cycle and a higher burnup level of nuclear fuel, it is necessary to increase the uranium enrichment. In a fuel assembly having a higher uranium enrichment, such an uneven power distribution is more pronounced. In order to flatten the power distribution in the axial direction and the radial direction, optimization of shape and arrangement of water rods, optimization of uranium enrichment distribution, partial change of fuel rod length, prevention of local power peaking at the initial burnup period with burnable poisons such as Gd, B, etc., and the like have been carried out.
All the above-mentioned techniques relate to the so-called element techniques. Even if some element technique is distinguished, a fuel assembly of higher burnup level cannot be obtained when the fuel assembly partially has some inconvenience. For example, Japanese Patent Application Kokai (Laid-open) No. 59-229475 discloses that irradiation growth and curving deformation can be prevented by controlling an Fl value as a crystallographic orientation parameter of a channel box to 0.15-0.5; but among the crystallographic orientation parameters a crystallographic orientation parameter in the normal-to-plate direction (Fr value) is most important. Furthermore, the fuel rods undergo irradiation growth and are elongated more than the initial length. As a result, the following inconveniences appear. Since the bottom ends of the fuel rods are fixed to the lower tie plate, the elongated fuel rods push the upper tie plate upwards. Since the top end of the channel box is fixed to the upper tie plate and the bottom end of the channel box is inserted into the lower tie plate, the channel box is pushed upwards due to the irradiation growth of the fuel rods, and at the final burnup stage the length of fitting allowance between the lower tie plate and the channel box is considerably decreased. In the nuclear fuel for high burnup level, the irradiation growth of the fuel rods is so large that the channel box is pushed upwards beyond the length of fitting allowance between the lower tie plate and the channel box. This has been a problem.